Field of the Invention
The invention relates to the use of structural steel tubes sheathed by an extruded layer made of a polyamide moulding composition in the foundation structure of offshore structures.
Description of the Background
Offshore structures are fixed structures erected offshore in open-sea locations. Examples of these offshore structures are wind energy systems, drilling platforms and lighthouses. Pipelines are not offshore structures for the purposes of the invention.
The foundation structure of offshore structures is the region that supports the actual functional unit. In the case of wind energy systems, the foundation structure supports the tower inclusive of turbine and rotors. In the case of drilling platforms, the foundation structure supports the platform inclusive of superstructure. In the case of lighthouses, the foundation structure supports the tower, if present, and also the lamp. The foundation structure is under water, in the intertidal zone, in the splash zone, or else sometimes in the aerosol zone. The foundation structure includes the foundation elements which anchor it in the sea floor.
As a consequence of the planned expansion of use of wind energy, a large number of offshore wind energy systems are planned for the coming years in the North Sea and also in other seas and in lakes. The entirety of the mechanical system of an offshore wind energy system is composed of the following components: turbines, rotors, tower and foundation structure.
Construction of foundations for these systems on the floor of the body of water, at locations which can be more than 100 km distant from the shore requires specific structural elements which differ greatly from those in the onshore sector. Some regions of these complicated structural elements, e.g. monopiles, jackets, tripods, tripiles, etc., have exposure to high static, and especially dynamic, loads, and also to high levels of corrosive attack. Factors that have to be considered, depending on the situation and depth of water at the particular location in question, are what is known as the 50-year wave, and also the tidal range. Other factors that have to be considered are intensive UV radiation, salt-water spray, spume, aerosols, temperature change, mechanical loads, colonization by molluscs and other organisms and the attendant mechanical erosion due to animal organisms, and also chemical erosion due to discharges or secretions from animal organisms and from other marine organisms. These structural elements use steel tubes, which for reasons of corrosion prevention either can have air-tight seals or can have a concrete filling. There can moreover be electricity lines or other supply lines running through the structural steel tubes.
The steel tubes required for structural elements have hitherto been designed with wall thicknesses substantially (up to 25%) greater than immediately necessary, and conventional coating materials are used for corrosion prevention here, mostly based on epoxy resin or based on polyurethane. These coating material systems do not generally provide any particular protection from mechanical loads.
WO 2009/027429 discloses the use of a metallic pipe having a surface sheathed by a polyamide layer, for producing a pipeline laid without any trench or sand bed. WO 2010/094528 discloses the use of a metallic pipe sheathed by an extruded layer made of a polyamide moulding composition, for producing a pipeline laid in water. However, in both cases the pipeline is not exposed to the combination of mechanical loads, corrosive attack and UV attack which is typical in the present invention and which includes, for example, severe wave impact.